Does your hair take forever to dry? Or maybe it seems like when you apply products, they seem to just sit on top without absorbing? Both are related to hair porosity, which can affect everything from washing to styling. Here’s what makes hair porous and why it matters.

What Is Porosity?

The science of how your hair behaves is complicated, but hair porosity has a pretty simple definition: how much water your hair can absorb.

The cuticle is the outside layer of your hair strand, and it's made up of overlapping cells, similar to shingles on a roof. These can let moisture in, and conversely, keep water out — depending on how porous your hair is.

Why Does It Differ?

Your natural porosity is hereditary. Other factors can contribute to porosity over time, which is why the hair near your scalp might not have the same porosity as the ends. Porosity increases when the hair cuticles experience damage, making them less able to protect the strand. Water can then more easily move in and out of the hair.

Chemicals, exposure to sun, heat styling and rough rubbing or brushing can all cause this type of damage.

How Porous Is Your Hair?

Low porosity hair tends to be shinier, less frizzy and holds onto hair color better. Because it doesn’t allow moisture in as easily, low porosity hair can be harder to moisturize. Often, people with darker hair have lower porosity.

High porosity hair tends to be less smooth, more stretchy and more prone to breakage. It takes longer to dry, but responds better to moisturizing treatments. Often, lighter hair is more porous.